1. Field of the Invention
This invention is concerned with devices, such as adjustable mirrors, smart windows, optical attenuators and displays, for controlling the reflectance and/or transmission of electromagnetic radiation.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing xe2x80x9csmart windowsxe2x80x9d with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. Such approaches are only partially effective since the window itself is heated so that heat is transferred into the interior by convection. In addition, these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable, low-voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications. For example, an effective means for controlling light transmission over a wide dynamic range is needed to permit use of inexpensive are lamps as light sources for projection displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing (i.e., cell gap) is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
Some earlier workers attempted to exploit reversible electrodeposition of a metal for light modulation, primarily for display applications [see for example, J. Mantell and S. Zaromb, J. Electrochem. Soc. 109, 992 (1962) and J. P. Ziegler and B. M. Howard., Solar Eng. Mater. Solar Cells 39, 317, (1995)]. In these cases, metal, typically silver or bismuth, was reversibly electrodeposited onto a transparent working electrode, usually indium tin oxide (ITO), from a thin layer of electrolyte sandwiched between the working electrode and a counter electrode. Both water and organic liquids (e.g., dimethylsulfoxide or dimethylformamide) were employed as solvents. The deposits obtained on the transparent electrode presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and were used to enhance light absorption by display elements. Pigments were often added to the electrolyte to provide a white background for improved contrast. An auxiliary counter electrode reaction (e.g., halide ion oxidation) was typically employed so as to provide a voltage threshold (which is needed for matrix addressing) and/or to avoid metal deposition on a transmissive counter electrode (which would offset the light modulation provided by metal deposition on the working electrode). Such auxiliary reactions introduced chemistry-related instabilities during long term operation and led to deposit self erasure on open circuit via chemical dissolution of the metal deposit. Nonetheless, the key drawback of reversible metal electrodeposition for display applications was the relatively slow response for attaining adequate light blocking.
A reversible electrochemical mirror (REM) device permitting efficient and precise control over the reflection/transmission of visible light and other electromagnetic radiation is described in U.S. Pat. Nos. 5,903,382, 5,923,456, 6,111,685 and 6,166,847, to Tench, et al. In this device, an electrolyte containing ions of an electrodepositable metal is sandwiched between a mirror electrode and a counter electrode, at least one of which is substantially transparent to the radiation. A typical transparent mirror electrode is indium tin oxide (ITO) or fluorine doped tin oxide (FTO) deposited on a transparent glass (or plastic) pane which serves as the substrate. Application of a voltage causes the electrodepositable metal, e.g., silver, to be deposited as a mirror on the mirror electrode while an equal amount of the same metal is dissolved from the counter electrode. When the voltage polarity is switched, the overall process is reversed so that the electrodeposited mirror metal is at least partially dissolved from the mirror electrode. A thin surface modification layer of a noble metal, e.g., 15-30 xc3x85 of platinum, on the transparent conductor is usually required to improve nucleation so that a mirror deposit is obtained. The thickness of mirror metal layer present on the mirror electrode determines the reflectance of the device for radiation, which can be varied over a wide range.
The REM technology can be used to provide control of either light reflectance or transmission, or both. A transmissive REM device suitable for smart window applications utilizes a counter electrode that is locally distributed, as a grid for example, on a transparent substrate, e.g., glass or plastic, so that mirror metal deposited thereon does not appreciably increase light blockage. In this case, high light transmission is provided by a locally distributed counter electrode of relatively small cross-sectional area and the device reflectance/transmission is adjusted via the thickness of mirror metal on the mirror electrode. As described in U.S. Pat. No. 6,166,847, such a transmissive counter electrode is not required for reflective REM devices used for adjustable mirror applications.
An electrolytic solution providing the inherent stability, high deposit quality, complete deposit erasure, long cycle life, and reasonably fast switching needed for most practical REM applications is described in U.S. Pat. No. 6,400,491, to Tench, et al. This solution is typically comprised of 1.5 M AgI and 2.0 M LiBr in a gamma-butyrolactone (GBL) solvent, and may also contain highly dispersed silica (HDS) added to produce a gelled electrolyte and/or dispersed carbon added to blacken the electrolyte so as to reduce background light reflection. Ionic liquid electrolytes may be used to provide faster switching and/or more uniform mirror formation and erasure in REM devices.
Under some circumstances, it would be highly advantageous to switch specific areas of optical modulation devices independent of other areas. For example, such localized switching of automotive mirrors could permit glare from headlights on following vehicles to be reduced without significantly affecting the image brightness in other areas of the mirror, which would provide a significant safety benefit. Likewise, localized switching of smart windows could provide improved visibility and/or increased interior lighting while retaining much of the energy benefit of such devices. In principle, localized switching of reversible electrodeposition devices could be provided by dividing the working electrode, which provides the optical modulation, into individually addressable segments. This is analogous to the approach generally used to switch display elements in a display device. However, the appreciable separation between segments required for electrical isolation and to accommodate the individual electrical connections is unacceptable for most optical modulation applications. Consequently, there is a significant need for a capability of providing localized switching of a substantially continuous optical modulation electrode. This capability might also be used to increase the performance and active area of display devices.
Reversible electrodeposition optical modulation devices are comprised of an optical modulation electrode, which is substantially transparent to the radiation to be modulated, a counter electrode, and an electrolyte containing electrodepositable metal ions disposed between and in contact with the two electrodes. Typically, both electrodes are planar and parallel and a seal is provided around the perimeter of the electrodes to contain the electrolyte and prevent intrusion of contaminants from the atmosphere. A negative applied voltage tends to cause metal deposition onto the optical modulation electrode and metal dissolution (or another electrochemical reaction) at the counter electrode, whereas a positive applied voltage tends to reverse these processes. Metal may be deposited on the optical modulation electrode as a mirror deposit that increases reflectance and decreases transmission of the radiation, or as a matte or rough deposit that decreases transmission and may increase light absorption. A typical device would employ an indium tin oxide (ITO) coating on a glass pane as the optical modulation electrode and a sheet or coating of the electrodepositable metal as the counter electrode. For a conventional display device, the optical modulation electrode is divided into segments (e.g., pixels or alpha-numeric elements) that are separately addressable via individual electrical contacts.
The present invention is a reversible electrodeposition optical modulation device employing a segmented counter electrode that enables individual areas of a continuous optical modulation electrode to be switched independently of each other. A voltage applied to a given counter electrode segment causes metal to be deposited or dissolved (depending on the voltage polarity) predominantly within the area of the optical modulation electrode directly opposed and defined by that counter electrode segment. Deposition or dissolution outside the defined area can be suppressed (to avoid overlap or cross-talk between display elements, for example) by utilizing a small electrode spacing and an electrolyte having relatively high electrical resistivity. The discontinuity between deposits on the optical modulation electrode resulting from the gap between counter electrode segments can be minimized (for an adjustable mirror or smart window device, for example) by utilizing a small spacing between counter electrode segments in conjunction with a relatively large cell gap and/or an electrolyte with relatively low resistivity.
One embodiment of the present invention is a reversible electrochemical mirror (REM) device employing an electrolyte (containing electrodepositable metal ions) in contact with a transparent mirror-forming electrode and a segmented counter electrode. The mirror electrode, which is the optical modulation electrode, is typically comprised of a thin surface modification layer of noble metal (e.g., platinum) on a layer of a transparent conducting oxide (e.g., indium tin oxide) on a glass or plastic substrate. The noble metal layer enhances nucleation so that mirror electrodeposits are obtained. The counter electrode in REM devices according to the present invention is a segmented sheet or layer of the electrodepositable mirror metal for devices that are designed to control radiation reflection, and is a segmented locally distributed electrode for devices that also transmit radiation. The reflectance of a selected area of the device is determined by the thickness of the mirror metal layer on the selected area of the mirror electrode, which can be adjusted by applying a voltage of the appropriate polarity between the mirror electrode and the segment of the counter electrode opposite to the selected mirror electrode area. The applied voltage causes mirror metal electrodeposition or dissolution predominantly within the selected area of the mirror electrode, while the reverse process occurs at the selected counter electrode segment. In this case, the discontinuity between deposits on the optical modulation electrode produced by adjacent counter electrode segments is minimized by utilizing a small spacing between counter electrode segments in conjunction with a relatively large cell gap and/or an electrolyte with relatively low resistivity. Applications for REM devices with segmented counter electrodes include automotive rear and side view mirrors with adjustable reflectivity and smart windows for use in transportation vehicles and buildings. Mirror deposits on the optical modulation electrode, typically obtained via use of a noble metal surface modification layer, are usually advantageous but the invention could also be applied to optical modulation devices that do not employ a surface modification layer.
Another embodiment of the present invention is a reversible electrodeposition display device employing a counter electrode having electrically isolated segments that are used to produce reversible metal electrodeposition in well defined areas, which serve as the display elements (e.g., pixels or alpha-numeric segments), on a continuous transparent display electrode. Cross-talk or overlap between adjacent display elements may be minimized by utilizing a small inter-electrode spacing and an electrolyte having relatively high resistivity, for example, an ionic liquid electrolyte. Poorly-reflecting electrodeposits may be used on display elements to block or absorb light, or a noble metal surface modification layer may be used to provide mirror deposits that reflect light. Reflective elements may be viewed directly or used for projection displays.
This invention enables metal deposition within selected areas of an optical modulation electrode in a reversible electrodeposition device, as well as relatively uniform deposition over the entire active electrode area with minimal discontinuity between the individually addressable areas. Localized switching of REM automotive mirrors permits glare from headlights on following vehicles to be reduced without significantly affecting the image brightness in other areas of the mirror. The enhanced mirror visibility is beneficial to driving safety. Localized switching of smart windows according to this invention may be used to provide improved visibility and/or increased interior lighting with minimal sacrifice in energy efficiency. For display devices, the invention permits switching circuitry and electrical contacts for the display elements to be placed on the counter electrode so that more pixel area is available on the optical modulation electrode.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.